The wind power industry has long seen three main approaches to the structural design and production of wind turbine blades:
1. Production of the blade by resin infusion using polyester or vinyl ester resin. In such blades, the shell and the cap layers of the structural box caps are made in a single infusion process, then joined with shear webs (see illustrations section). The aerodynamic shell and the structural box caps are not independent parts, and the whole body is infused with the same resin. Such structure has the disadvantage that the inferior fatigue and strength properties of the lower cost resins mean that more volume of laminate must be used, resulting in a heavier blade. Further, such structure has the disadvantage of possessing areas of critical structural bonding, placing a high requirement on the strength of the bonding adhesive and good preparation of the surfaces to be joined, by grinding or other roughening techniques. Such structure requires well trained and monitored workers to prevent the development of quality defects during the production process. The process is prone to occurrence of air inclusions in the resin, wrinkles in the dry fiberglass, oversized or poorly matched structural bond lines, and failed structural bonding due to surface contamination or insufficient surface roughening. These common faults have been sources of numerous blade failures in service over the past 2 decades.
2. Production of the blade by resin infusion or hand lamination, using epoxy resin. In such process, the cap layers of the structural box are produced in a first ‘prefab’ step, then the ‘prefab’ is laid into the structural shell, which subsequently completely envelopes the ‘prefab’ parts. The aerodynamic shell and the structural box are not independent parts. Such structure has the disadvantage of using more expensive epoxy resin throughout, while achieving only mid-level fatigue and strength properties. Further, such structure shares all the disadvantages of method 1, regarding the structural bonding and quality control problems. The only real difference from method 1 is a use of a more expensive resin matrix to generate some weight savings.
3. Production of the structural box in a single piece by laying epoxy-fiberglass and/or epoxy-carbon fiber prepreg layers onto a box shaped male mandrel. A separate non structural shell from similar type epoxy based prepreg material is formed in two halves using other moulds. Then The shells and the structural box are bonded together. However, as the shell is not a load carrying member, but merely an aerodynamic fairing, the joint is not a load carrying structural bond. The standards for joint preparation and adhesive strength can therefore be relaxed considerably. This production technique eliminates much of the quality control risk associated with methods 1 & 2. Air inclusions and wrinkles can be eliminated altogether, while the issue of bonding quality becomes much less important. However, this approach results in considerably higher cost, since costly multi-axial prepreg is also used for the non-structural shell, where superior structural properties provide little benefit. Further, because the prepreg shell material requires a higher curing temperature, more expensive high temperature resistant SAN (styro acryl nitrile), PVC (poly vinyl chloride) or PET (polyethylene terephthalate) foam core must be used within the blade shell sandwich laminate, in place of the balsa wood or low cost foam preferred for methods 1 & 2. Also, the shell moulds need to be capable of higher temperatures in order to cure the prepreg material, adding additional cost.